JDAM/JSOW elevation in AGL

[backspace340]

that 1' above ground on your house is always going to be 1' above ground in that particular location of the house. the map has been mapped long time ago. and that "hemorrhoid" :lol: of the earth has been incorporated figuring out GPS coordinates. what is a bad idea is incorporating MSL to the equation because you cannot accurately get that value specially in enemy air space. like I said you cannot ask the airbase you're about to bomb what their barometric pressure at the moment

 

The Hornet supposedly only got elevation data with TAMMAC in Lot 23, which isn't modelled in our version of the Hornet. So while your fitbit might know the elevation of every spot in the world, the Hornet being modelled doesn't.

[RED]

I think there are different misconceptions by different people.

 

JDAMs are INS guided. However through GPS updates the INS unit's drift will be compensated. JDAMs work much like the INS+GPS systems in the F/A-18 while the technical solutions and their performance are different (big airplane/small kit on the bomb).

 

Like you probably learned in school, you can describe 3d space by 3 independent vectors. If you get a value for each vector you will have a defined position in relation to your vector system. Like drPhibes wrote, the modelused for the weapon system is made to be easily used on earth. It is a mathematical abstraction and not a map. But you will still be able to describe a position in reference to your model by 3 values (Lat/Long/elev).

 

The bomb only knows at what angle it should arrive at what position in reference to that model. By entering a position and elevation into the system we are inputting an already translated position. The step from real word to model of the system was already done and doesn't need to be made by the system.

 

I am not an expert on this. There are probably many people with better info on weapon systems.

[Revs]

The Hornet supposedly only got elevation data with TAMMAC in Lot 23, which isn't modelled in our version of the Hornet. So while your fitbit might know the elevation of every spot in the world, the Hornet being modelled doesn't.

 

you do not need elevation value for everything, the one guy said that there is a hemorrhoid values that the system can factor in the calculations. you still get your elevation from a known elevation from a map/tgp with ranging/or a guy with a fitbit on the ground.

[Revs]

I think there are different misconceptions by different people.

 

JDAMs are INS guided. However through GPS updates the INS unit's drift will be compensated. JDAMs work much like the INS+GPS systems in the F/A-18 while the technical solutions and their performance are different (big airplane/small kit on the bomb).

 

Like you probably learned in school, you can describe 3d space by 3 independent vectors. If you get a value for each vector you will have a defined position in relation to your vector system. Like drPhibes wrote, the modelused for the weapon system is made to be easily used on earth. It is a mathematical abstraction and not a map. But you will still be able to describe a position in reference to your model by 3 values (Lat/Long/elev).

 

The bomb only knows at what angle it should arrive at what position in reference to that model. By entering a position and elevation into the system we are inputting an already translated position. The step from real word to model of the system was already done and doesn't need to be made by the system.

 

I am not an expert on this. There are probably many people with better info on weapon systems.

 

 

It's mostly what I've been trying to say lol. I got ideas but don't know how to explain properly

[dorianR666]

revs, what you describe would require hornet to have terrain elevation database for the theatre. as far as i know, our hornet version doesnt have terrain database.

 

which means that setting AGL makes no sense.

 

 

what red describes may be correct, but that would still need MSL or WSG84-relative elevation, not AGL, if we dont have terrain elevation database onboard.

Edited June 18, 2019 by dorianR666

[backspace340]

you do not need elevation value for everything, the one guy said that there is a hemorrhoid values that the system can factor in the calculations. you still get your elevation from a known elevation from a map/tgp with ranging/or a guy with a fitbit on the ground.

 

And what would the map / tgp with ranging / guy with a fitbit tell you to help input the elevation in the Hornet? If they say they're at AGL 0, how does that help you?

[Revs]

And what would the map / tgp with ranging / guy with a fitbit tell you to help input the elevation in the Hornet? If they say they're at AGL 0, how does that help you?

 

you all just made me realize I do not know what I am talking about, I'll shut up now. goodnight :pilotfly:

[EcceHomo]

JDAM, latitude and longitude must be defined in WGS-84 coordinate system with elevation given by MSL or HAE. H, h, N in the graph represent the relationship between them.

h=HAE

H=MSL

N=Geoid Height

[Nospin44]

JDAM, latitude and longitude must be defined in WGS-84 coordinate system with elevation given by MSL or HAE. H, h, N in the graph represent the relationship between them.

h=HAE

H=MSL

N=Geoid Height

 

In the bug report that started this discussion, Wags and Nineline confirmed that elevation is currently using AGL. But I, like others are confused because that doesn't seem to make sense.

 

So to clear things up. Is AGL as the elevation format incorrect?

[EcceHomo]

In the bug report that started this discussion, Wags and Nineline confirmed that elevation is currently using AGL. But I, like others are confused because that doesn't seem to make sense.

 

So to clear things up. Is AGL as the elevation format incorrect?

 

AGL needs to be converted to MSL or HAE. It's no problem if it's MC automatic conversion. If there is no conversion, that would be wrong. In UFC input, DCS lacks pages to enter HAE. It's like SMS lacks wind OSB.

[Harker]

I read the relevant bug thread and the answer is confusing. I think the devs meant that you need to set the elevation to 1ft above AGL.

 

GPS uses global data, so N,E and ASL elevation. AGL doesn't mean anything to it. We don't have TAMMAC and thus, no way to accurately translate AGL to ASL. We also don't have a way to get AGL currently, due to the lack of A/G radar. In the A-10C, you can mark a point on the ground and get coordinates and elevation, because it has TAMMAC.

 

And regarding the barometric pressure that I saw mentioned, what does it have to do with a GPS point in space? It's simply so that the Hornet's altimeter, which is based on barometric pressure, displays the correct value. But I find it hard to believe that anyone would use BARO altitude to calculate the AGL in any meaningful way.

[RED]

But I find it hard to believe that anyone would use BARO altitude to calculate the AGL in any meaningful way.

 

Psst. Try to hide from Viggen drivers and Saab, they want to have a word with you. :megalol:

[Harker]

Psst. Try to hide from Viggen drivers and Saab, they want to have a word with you. :megalol:

Never had the pleasure of flying it. They use BARO alt for targeting? So they need to know the barometric pressure at the target point and make sure it doesn't change during the mission. I don't know, it sounds very prone to inaccuracy to me. Care to explain how it manages to work reliably?

[Ramsay]

We don't have TAMMAC and thus, no way to accurately translate AGL to ASL.

The TAWS (Terrain Awareness Warning System) uses TAMMAC as one of it's data sources.

 

 

Why do you think the DCS Lot 20 Hornet does not have TAMMAC ?

Edited June 18, 2019 by Ramsay
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[Harker]

The TAWS (Terrain Awareness Warning System) uses TAMMAC as one of it's data sources.

Why do you think the DCS Lot 20 Hornet does not have TAMMAC ?

I had a discussion about it with NineLine (https://forums.eagle.ru/showthread.php?t=242412&highlight=tammac). Indeed, it's more correct to say that we don't know for now. He said he'll ask Wags about it. I know TAWS relies on DTED, which is part of TAMMAC AFAIK. A Lot 20 Hornet fresh off the production line won't have TAMMAC, in 1998. Lot 23 Super Hornets were the first to come with TAMMAC and the JHMCS, in 2001 (https://forums.eagle.ru/showpost.php?p=2919850&postcount=760).

 

Of course, our Lot 20 will have received upgrades from its delivery in 1998 to 2005. It's a question of the software package (Operational Flight Program - OFP) installed. We have OFP 13C, which I think it's not the latest OFP in 2005, but it's entirely possible that an active duty F/A-18C had it back then. The thing is that I can't determine whether OFP 13C included TAMMAC or not, but I think it did not.

 

ED has said that they use OFP 13C as the base, since they have the info for it and they might include some features from later OFPs, if they deem it necessary and data is available. The inclusion of TAWS (and possibly TAMMAC) might be related to that (or it might be in OFP 13C and I might be wrong). Either way, I'd welcome TAMMAC very much.

[Flagrum]

The TAWS (Terrain Awareness Warning System) uses TAMMAC as one of it's data sources.

 

 

Why do you think the DCS Lot 20 Hornet does not have TAMMAC ?

That would help to determine the proper elevation of the target. But we are discussing if the weapon itself requires an absolute elevation or if a relative elevation of "a bit above the ground" could suffice (i.e. changing manually the ELEV of the weapon).

 

 

 

Assuming that the weapon does not have it's own elevation database, it has no idea where the ground is.

[Ramsay]

That would help to determine the proper elevation of the target. But we are discussing if the weapon itself requires an absolute elevation or if a relative elevation of "a bit above the ground" could suffice (i.e. changing manually the ELEV of the weapon).

 

Sorry, I took it as a given that a JDAM/JSOW would need absolute target positional information i.e. Lat. Long. and Altitude (MSL (mean sea level) or HAE (height above ellipsoid)).

 

I thought this was more a Pilot Vehicle Interface question as to whether, if a pilot entered target height as AGL, the MC (mission computer) could convert to MSL/HAE for the weapon.

 

Although I'm surprised the DCS Hornet uses AGL on it's JSOW weapon page, it's possible the Lot 20 uses DTED (not always available) or has it's own peculiarities.

 

AFAIK JDAM/JSOW use HAE for their internal guidance algorithms.

 

Assuming that the weapon does not have it's own elevation database, it has no idea where the ground is.

Correct, without GPS (weak signal/jamming), the JDAM/JSOW get their initial INS alignment from the launching aircraft and guide to the targets relative position.

 

AFAIK JDAM/JSOW do not have DTED.

[Flagrum]

I thought this was more a Pilot Vehicle Interface question as to whether, if a pilot entered target height as AGL, the MC (mission computer) could convert to MSL/HAE for the weapon.

 

Although I'm surprised the DCS Hornet uses AGL on it's JSOW weapon page, it's possible the Lot 20 uses DTED (not always available) or has it's own peculiarities.

Yes, it could be a matter of the user interface. But that would seem strange to me. "Elevation" is, imo, a rather unusual term when talking about AGL.

 

 

Also I believe, the JDAM/JSOW page is the weapon data, i.e. as it is stored in the weapon. You don't set up an "abstract" delivery profile for your attack, but you rather program each single weapon individually and directly.

 

 

And finally, the elevation field on the JDAM/JSOW page references the WGS ellipsoid. Why would you do that if you mean "AGL"?

 

 

Some clarification from 9L or Wags would help here - as they brought up the idea to enter "AGL" as elevation ...

[Tj1376]

I just got a notification from 9L that they have confirmed this is a bug and have reported it to the team. If you go back to the bugs section you can see the update.

 

TJ

 

 

Sent from my iPhone using Tapatalk

[sobek]

Entering AGL makes no sense at all. Why would i want to steer my bomb to a coordinate above the surface? I would have little control over where that bomb actually hits.

 

You always want to enter coordinates that are relative to your origin of coordinates, not enter two coordinates relative to your origin (lat & lon) and then enter one relative to a function of lat & lon. That makes no sense whatsoever.

[Tj1376]

Entering AGL makes no sense at all. Why would i want to steer my bomb to a coordinate above the surface? I would have little control over where that bomb actually hits.

 

You always want to enter coordinates that are relative to your origin of coordinates, not enter two coordinates relative to your origin (lat & lon) and then enter one relative to a function of lat & lon. That makes no sense whatsoever.

 

 

The post directly above yours reads:

 

I just got a notification from 9L that they have confirmed this is a bug and have reported it to the team. If you go back to the bugs section you can see the update.

 

TJ

 

 

Sent from my iPhone using Tapatalk

 

 

 

Sent from my iPhone using Tapatalk

[sk000tch]

Entering AGL makes no sense at all. Why would i want to steer my bomb to a coordinate above the surface? I would have little control over where that bomb actually hits.

 

You always want to enter coordinates that are relative to your origin of coordinates, not enter two coordinates relative to your origin (lat & lon) and then enter one relative to a function of lat & lon. That makes no sense whatsoever.

 

There are many reasons, terminal trajectory and air burst height can be tailored for fragmentation patterns, or to impact a structure at a certain point/trajectory. I don't want to get in trouble again so just google JDAM air burst, or fragmentation or low collateral damage. There's plenty of info.

 

As to question at hand, some questionable info in this though more recent posts have it right. Maps and elevation data is, at least in 2005, not of sufficient accuracy for JDAM targeting. Thus, the only time AGL is used is with certain fuzes.

 

To answer the question, regardless of mode, elevation is entered in reference to zero surface (HAE or MSL). From JDAM menu, selecting TGT UFC, ELEV on UFC may be input in either MSL or WGS, in either feet or meters.. Both in reference to a zero point, not AGL. HAE, height above ellipsoid, is just the zero point for WGS 84 geometric/ellipsoid model of the earth (geodetic system used by GPS system). MSL is of course Mean Sea Level, which again is a reference to zero point, though an approximation and less precise. HAE is recommended for use in our lot, but i suspect truncation converting HAE to MSL and MSL decreased precision won't be modeled so wouldn't stress it.

 

Interestingly, offset is entered as absolute elevation, not differential relative to the offset reference point. And again, several fuzes have proximity sensors that allow AGL, but that's an answer to a different question.

[theinmigrant]

Right now the elevation entry doesn't seem to do anything so long as it's set > 0.

 

For a target located at 3,000 feet MSL, I set one bomb at MSL, I've set one at 1 foot (assuming ASL), and a third at 5,000 feet (deliberately wrong). All bombs impacted within twenty feet of each other.

 

I'd assume this is WIP, because this can not be how the weapon functions in reality. I find it difficult to believe the F/A-18 has a terrain database accurate enough at the resolution required for that sort of accuracy.

 

I wonder why. I really thought that the JDAM/JSOW data input was simulated. I didnt try but as you say it wont matter what we enter.

[backspace340]

I wonder why. I really thought that the JDAM/JSOW data input was simulated. I didnt try but as you say it wont matter what we enter.

 

I've definitely seen elevation matter with JDAMs/JSOWs, so a bit perplexed by that too.

[EcceHomo]

The Elevation of TGT/ORP, o/s are MSL or HAE (WGS). The source of input values is distinguished by MSL and WGS. This is not a complicated problem.

Now it should be WIP. Errors are understandable. Dynamic launch zones, loading PP plans from MUMI, which is what we expect.